Maternal aggression in the laboratory rat is an important component of maternal behavior; its onset is correlated with the onset of other items of maternal behavior (e.g., nursing, retrieving and licking pups) and its decline parallels their decline. Since these items have been shown to be based upon hormonal stimulation, specifically estrogen stimulation on a background on estrogen/progesterone priming, the present study will determine whether maternal aggression is also based upon this sequential pattern of hormone stimulation. Nonpregnant females will be hysterectomized and ovariectomized and given a priming regimen of estrogen alone, progesterone alone, both hormones or neither and tested for maternal aggression with an intruder male after 16 days of treatment followed by an otherwise subthreshold estrogen dose. In addition to hormonal stimulation, maternal aggression is strongly influenced postpartum by pup stimulation: in the mouse suckling stimulation is crucial for maternal aggression. To test whether this is also the case in the rat several different methods will be used either to eliminate suckling or to interfere with its hormonal effects postpartum, and to prevent the prepartum onset by preventing the prepartum release of prolactin.